1. Field of the Invention
The present invention relates to nanometer-size spherical particles having a size of about 1 μm or less, a sphericity of about −10% to +10%. The present invention also relates to nanometer-size spherical particles having a composite structure. The present invention also relates to a method for producing such particles.
2. Description of the Related Art
Particles of various metals, alloys, metal oxides, metal nitrides, metal silicides, or mixtures of these materials have been used for various applications such as magnets, catalysts, electrode materials, battery materials, cold insulators, fire-resistant materials, and sintered metal materials.
These materials have been researched to improve their properties. The research has shown that the use of fine particles having good sphericity and a nanometer size is effective in enhancing their properties. For example, normal copper particles have a melting point of 1,083° C.; however, nanometer-size copper particles are known to have a melting point of about 200° C. Normal silver particles have a melting point of 962° C.; however, silver particles with a size of about 20 nm are known to have a melting point of about 150° C. It is known that amorphous materials have properties inferior to those of crystalline materials. In consideration of the industrial use of the nanometer-size spherical particles, the yield (the percentage of the amount of nano-particles having a target size with respect to the amount of input raw materials) of a final product must be improved.
In conventional methods, the sphericity of the nanometer-size particles is inferior. The particles with inferior sphericity can be readily oxidized; hence, these particles are unsuitable for practical use. Since crystal structures are broken during the reduction in particle size, only fine amorphous particles can be obtained by the conventional methods. In some situation, particles including a crystalline region are useful in view of good properties.
The microscopic structure of particles is one of factors in determining properties of the above materials along with the composition, shape, and size thereof. Recently, particles whose microstructure consists of two or more regions or components (composite structure) have been focused on and researched for various applications.
Japanese Unexamined Patent Application Publication No. 2005-320195 filed by the inventors of the present invention discloses a method for producing silicon particles having a polycrystalline structure, including a composite structure.
Japanese Unexamined Patent Application Publication No. 2005-320195 discloses that the silicon particles have a composite structure and a particle size of 50 nm to 10 μm. The silicon particles have no spherical shape but an irregular shape. A final product obtained by the method is limited to the silicon particles. The publication does not disclose that the method can be used to produce fine particles of various metals, alloys, metal oxides, metal nitrides, metal silicides, or mixtures of these materials. The publication discloses amorphous structures but does not disclose crystalline structure.
Japanese Unexamined Patent Application Publication No. 2004-35398 discloses a method for producing fine particles by plasma-treating a starting material. The starting material is plasma-treated in such a manner that the starting material is placed outside an arc flame. These fine particles have a diameter of 100 nm to 20 μm. These fine particles have a wide size distribution. Furthermore, these fine particles have low sphericity and the yield thereof is insufficient for industrial use.
U.S. Pat. No. 6,808,568, owned by the inventors of the present invention, relates to particles having a composite structure and a sphericity of about −10% to +10%. The disclosed method can form completely spherical particles. The patent discloses metal particles having a composite structure. The metal particles are those having a size of 1 to 300 μm, that is, micrometer-size spherical particles. In the specification of the present invention, the particles having such a size are hereinafter generically referred to as an intermediate product. However, the US patent does not disclose any nanometer-size particles.
In conventional methods, nanometer-size particles having a sphericity of about −10% to +10% cannot be produced.